A brushless synchronous power generation apparatus is configured by combining a synchronous generator, an AC exciter, and a rotary rectifier. A static frequency converter (hereafter abbreviated to SFC) is connected to a stator armature winding of the synchronous generator, in which is applied an SFC starting system which changes the frequency of armature current of the synchronous generator and puts the synchronous generator in starting operation as a synchronous motor.
On the other hand, when a turbine and synchronous generator rotor shaft used in a power plant with a steam or gas turbine or the like is left remaining in high-temperature state and in non-rotating state while in non-operation, shaft bending occurs due to thermal strain or to the self-weight of the rotor. In order to prevent the shaft bending, a turning operation, which causes the turbine and generator rotor shaft to rotate at low speed (about two to ten r/min) for a certain time after the non-operation, is carried out by using a turning device formed of, for example, a drive motor and a gear.
In general, two kinds of systems: a turning motor and an SFC starting system, are installed in the device which drives the turbine and synchronous generator rotor shaft, and are used in different ways. The SFC starting system is used for an operation which causes an increase in speed from a turning rotation speed of 3 r/min to a gas turbine ignition rotation speed of 2000 to 2400 r/min, and the turning motor is used for a turning operation in which the turning rotation speed increases from no rotation of 0 r/min to 3 r/min and, after reaching 3 r/min, remains at 3 r/min for a certain time.
An SFC starting device (a 120-degree energizing current type inverter) is such that in a low rotation speed region, for example, at a turning rotation speed of 3 r/min, SFC current is intermittently supplied, leading to a decrease in SFC current control accuracy. Because of this, control at a low rotation speed of several r/min is difficult with the SFC starting device, and in general, the SFC starting device is not suitable for the turning operation. Also, when the turning motor is started from no rotation to a gas turbine ignition rotation speed of 2000 to 2400 r/min, the capacity and the whole configuration of the turning motor are extremely large, which proves not to be economical, so it is common that a rotor shaft drive is of a combination of the turning-dedicated gear and motor and the SFC starting.
A power generation system is such that direct current is supplied to a rotor field winding of the synchronous generator, and a magnetic flux generated from the rotor winding is interlinked with the stator armature winding, as a result of which an induced electromotive force is generated in the armature winding, thereby carrying out power generation. As the technique of supplying direct current to the rotor field winding, there are two kinds: a “slip ring system which brings carbon brushes into mechanical contact with steel rings” and a “brushless system which uses a rotating machine on which a three-phase full-wave rectifier circuit is mounted”.
A brushless exciter is of a structure in which an armature winding is wound on the rotor, while a field winding is wound on the stator, and the rotor armature winding of the brushless exciter and the rotor field winding of the synchronous generator are on the same rotating shaft. Because of this, a current voltage induced in the rotor armature winding of the brushless exciter is rectified by a rotary rectifier of three-phase full-wave rectification, and direct current is supplied to the rotor field winding of the synchronous generator.
In the case of a power plant in which a gas turbine and a synchronous generator are combined, the gas turbine is short of torque as a prime mover and has a rotating region in which the shafting cannot be increased in speed, so that the SFC is connected to the stator armature winding of the synchronous generator, and the frequency of armature current of the synchronous generator is changed, thus putting the synchronous generator in starting operation as a synchronous motor.
When in starting operation, the synchronous generator is operated as the motor, but in the same way as in the operation as the generator when at rated speed, it is necessary to supply direct current to the synchronous generator field winding.
In a brushless excitation system, when DC field current is applied to the AC exciter field winding, no induced voltage is generated in the AC exciter armature winding when the rotor shaft is stopped, and it is difficult to supply the field current to the synchronous generator field winding. Also, in the state of a low rotation speed of several r/min, there is the problem of a short supply of direct current to the synchronous generator field winding.
With respect to the problem, PTL 1 proposes a system wherein dq-axis two-phase AC windings are used as the stator field winding of the AC exciter, and the d-axis winding and the q-axis winding are AC excited by an inverter at a phase difference of 90 degrees, thereby enabling a rotating field to be generated from the stator, thus enabling a required DC output to be generated even when rotation is stopped.
Also, as for a turning device formed of, for example, a drive motor and a gear, a configuration which is high in reliability without using the gear is desired in light of a problem such as the maintenance of a mechanical device, and a survey of the configuration has found that PTL 2 proposes a configuration such that a brushless excitation device, being configured of a first AC exciter which excites with a DC power source from a stator side and a second AC exciter of a wound rotor induction motor-driven type which excites with a three-phase power source from the stator side, is disposed on the same shaft as a synchronous generator, wherein a first rotary rectifier is connected to an armature winding of the first AC exciter, a second rotary rectifier is connected to a secondary winding of the second AC exciter, and the first rotary rectifier is connected in series to the DC side of the second rotary rectifier, thus supplying excitation current to the field winding of the synchronous generator.